[clang] [LifetimeSafety] Cache lifetimebound macro lookup. NFC. (PR #205250)

[via cfe-commits]

https://github.com/NeKon69 commented:

Yay, mostly looks good, left a few questions.
Also:
> | Case | Before [359bfe6](https://github.com/llvm/llvm-project/commit/359bfe62ed1b1cd498cee558ed8699b2b88ae58b) | [359bfe6](https://github.com/llvm/llvm-project/commit/359bfe62ed1b1cd498cee558ed8699b2b88ae58b) | After Cached |
> |---|---:|---:|---:|
> | C++ 200 diagnostics, macro passed via command line option | 24.8 ms | 24.6 ms | 29.3 ms |

Well that's a bit weird.

https://github.com/llvm/llvm-project/pull/205250